It is not the fact it is an unknown picture of her. It is the fact that it is a NUDE or TOPLESS picture. It is inappropriate and it gives off the wrong image of Kate Middleton as to who she really is. The article may have offended her as a person as a result. If anything, a person should get consent for it.
Yes, I believe that the French magazine "Closer" crossed the line by posting topless pictures of Kate Middleton. It isn't so much an issue because of the fact that she is part of the Royal Family, but because everybody deserves their privacy and shouldn't have to worry that every private moment of their life is being photographed. People argue that because they are in the Royal Family and because celebrities are in the "public spotlight" that they are not entitled to the same privacy as everyday people are, but this sounds to come from envy rather than genuine belief that popular people should have no right to privacy.
It was a clear violation of privacy in the manner if one takes note of the manner in which the photos were obtained. it's not like she was wandering through the city nude or something, they were spying on her (at least that's what the BBC told me). That is, in all aspects, a violation of privacy and a definite crossing of the line.
Naked pictures of someone are something that should not be published without a person's consent, regardless of who they are. The fact that someone was spying on Kate Middleton and her husband should be illegal in and of itself, but then to take pictures and print them, it's even more uncalled for. The magazine and the photographer should be prosecuted for this.
She's not Paris Hilton or Kim Kardashian that benefit from those sort of stuff, she's part of the royal family, have some respect. She was on vacation and she did it in private with only her husband there. That's going a bit far
They didn't cross the line, it is her fault for doing something stupid and she is full aware how much the paparazzi love her.
A simple Paparazzi scandal where some celebrity got caught... big deal it happens all the time. She's also suing them over it.